This invention relates to a battery operated power supply for an electroluminescent (EL) lamp and, in particular, to an EL driver having a lamp discharge detector for controlling low frequency switching.
An EL lamp is essentially a capacitor having a dielectric layer between two conductive electrodes, one of which is transparent. The dielectric layer may include a phosphor powder or there may be a separate layer of phosphor powder adjacent the dielectric layer. The phosphor powder radiates light in the presence of a strong electric field, using very little current. Because an EL lamp is a capacitor, alternating current must be applied to the electrodes to cause the phosphor to glow, otherwise the capacitor charges to the applied voltage, the current through the EL lamp ceases, and the lamp stops producing light.
In portable electronic devices, automotive displays, and other applications where the power source is a low voltage battery, an EL lamp is powered by a driver that converts direct current into alternating current. In order for an EL lamp to glow sufficiently, a peak-to-peak voltage in excess of about one hundred volts is necessary. The actual voltage depends on the construction of the lamp and, in particular, the field strength within the phosphor powder. The frequency of the alternating current through an EL lamp affects the life of the EL lamp, with frequencies between 200 hertz and 1000 hertz being preferred. Ionic migration occurs in the phosphor at frequencies below 200 hertz. Above 1000 hertz, the life of the phosphor is inversely proportional to frequency.
The prior art discloses several types of drivers including an inductive boost circuit having an inductor in series with a switching transistor. Current through the inductor causes energy to be stored in a magnetic field around the inductor. When the current is abruptly shut off, the induced magnetic field collapses, producing a pulse of high voltage. The voltage across the inductor is proportional to L.multidot..sup..delta.i /.sub..delta.t. Thus, a low voltage at high current is converted into a high voltage at low current. The voltage on the lamp is pumped up by a series of high voltage pulses from the boost circuit.
The direct current produced by the boost must be converted into an alternating current in order to power an EL lamp. It is known in the art to switch the inductor in such a way as to produce alternating current on a single output; see U.S. Pat. No. 5,313,141 (Kimball). The current through the lamp alternates at a low frequency (200-1000 hertz).
It is known that an EL lamp can produce acoustic noise when electrical pulses are applied to the lamp due to the abrupt discharge of the lamp when polarity is reversed. The abrupt discharge causes a current spike to flow through the lamp that shortens the life of the lamp. It is also known to control the discharge current through an EL lamp to minimize noise generated by the lamp; e.g. U.S. Pat. No. 5,789,870 (Remson) and U.S. Pat. No. 6,038,153 (Andersson et al.).
The prior art uses controlled current devices for discharging a lamp for a minimum period of time, thereby reducing the noise generated by an EL lamp. A problem with these devices is that the discharge time is derived from a clock signal produced locally in an integrated circuit or coupled to the integrated circuit from another source. As an EL lamp ages, the capacitance of the lamp decreases, thereby reducing the time necessary to discharge the lamp, but the time available for discharge does not decrease. Thus, the lamp is "off" for slightly longer periods and the lamp dims even more than due to aging alone.
In view of the foregoing, it is therefore an object of the invention to provide an optimum discharge cycle for an EL lamp driven by a battery powered driver.
Another object of the invention is to improve the life of an EL lamp driven by a battery powered driver.
A further object of the invention is to provide a detector circuit for monitoring the discharge of an EL lamp during each half cycle of the low frequency signal.